1. Field of the Invention
The present invention relates to endoscopes, more, specifically, to a wide angle endoscope in which an observation optical system has a wide viewing angle.
2. Description of the Related Art
As well known in the art, endoscopes have been widely used in the medical field and the like. The endoscope is able to observe the organs inside the body cavity by inserting an elongated insertion unit into the body cavity, or to perform various treatments using a treatment instrument inserted into an insert channel for the treatment instrument as necessary.
A bendable part is arranged at the distal end of the insertion unit, and the observing direction of an objective lens at the end of the observation optical system arranged in the insertion unit can be changed by operating the operating unit of the endoscope to bend the bendable part.
The viewing angle of the observation optical system of the conventional endoscope is about 140°, for example, and the operator observes the body cavity with the observation image of the relevant viewing angle, but when desiring to observe a region outside the field of view, the operator bends the bendable part to observe the region outside the field of view, as described above.
However, when observing the inside of the large intestine and the like, the desired observation image of the back side of the lamella of the large intestine and the like cannot be obtained by simply bending the bendable part. In view of such situation, an endoscope having a wider viewing angle to allow the observation of a wider range is proposed (see e.g., Japanese Patent Application Laid-Open (JP-A) No. 2001-258823).
The endoscope proposed in Patent Document 1 has the observation optical system having a wide viewing angle, e.g., 180° arranged at the bendable part and the distal end portion of the insertion unit. Further, when the observation optical system is made to have a wider angle, an illumination optical system, arranged at the bendable part and the distal end portion of the insertion unit and illuminating the body cavity, is arranged on an axis slanted with respect to an axis direction where the observation optical system is arranged for even and thorough illumination of the body cavity.
Further, the illumination optical system illuminates the observing range of wide angle of the observation optical system with a sufficient brightness and light distribution, and thus is generally configured with a plurality of illumination lenses (see e.g., JP-A No. 10-99268).